[ACMP investiupgradifications] All things PREAMP

[Paul G]

Hello all:

I've been following this thread without joining--really appreciative of all the investigations you all are doing on these preamps--but I wanted now to ask a question...

I know that judging the usefulness or even quality of gear is tough to impossible, given that the application of a given tool is what matters, anything can be the "right" tool for a give source/song.  That said, I'm a seriously unknowledgeable person when it comes to the insides of gear and so I'm trying to get a sense of how "worth it" it is to send my two 81s and two 73s into Steve Hogan for his upgrades.  Steve has been super gracious and helpful in emails I've sent to him--explaining all the things he can do to upgrade these pres--but I still don't have an overall sense of whether or not to invest more dough in these or sell them off and put the entire cost of the the four towards one higher end pre.

I suppose my question is this simple: once Steve Hogan does his upgrades and I've invested around 500 bucks or so total in each of the pres, is that still a fairly sweet price for each of these channels?  I.e., once Steve rips out the crappy parts and adds better ones, is it likely that these pres will be nice additions to a project studio?  To give context for my pre selections, I have the Sytek 4-channel thing (with burr-brown on channels 3&4), a JoeMeek channel strip, and two channels of racked PM-1000s.  I was hoping the ACMPs would be a nice change of pace for vocals and drums, but I could get just one channel of a higher-end pre to use mainly on vocals..

Anyway--hope this is an appropriate question for this particular forum.  I know this is about investigating these pres, but I figured maybe there were other folks lurking here who, like me, always assumed we'd have someone mod these pres, but aren't clear how good they'd be post-mods.

thanks a bunch!
mp

I'm doing a "Steve Hogan" mod to a 73 myself and have just hooked my two brothers up with a 73 each.  Their two units are being shipped to Steve for mods.  They will have a little more into their units than I but we're hoping that we'll end up with a great sounding pre.  It's really a crap shoot.  My 73 doesn't sound bad now, (I rotated the tranny) but it's still noisy.  Too much so for vocals and that's what I really wanted this pre for. 

One thing you need to consider though, once you're finished with the mods, what would this unit be worth if you had to sell it.  The TNC gear has gotten a bit of a black eye and to figure you'll have $500 - $600 in the unit, it may be a hard sell to get that out of it.  Just a thought.

Paul ;D

 

[crazydoc]

As far as I know, nobody knows what these will sound like when they are fully modded by Steve. 

I'm sure somebody will correct me if I'm wrong (and please do) but my take on all this is that any mods being done will not affect the "sound" of these pres to any appreciable degree.

To my mind, the changes to these can be divided into two categories:

1) Making them usable. This would entail getting rid of the inductor hum, the gain switch blast, and, on the 81, the noise derived form the transistor oscillation on the EQ boards. Whether these are best done by power transformer upgrade, inductor upgrade, shielding, transistor replacement or other, remains to be seen.

2) Making them last. This would entail upgrading some parts, mostly in the power supplies - in particular capacitor capacitance and voltage ratings.

As far as improving or changing the sound or coloration of the preamps, some of Steve's suggestions will probably have some minor beneficial effects, such as changing the values of coupling caps, reworking the gain switch resistors and removing the 2.2nF cap from the line input transformer (and possibly the cap on the mic input also.). But I think in general, the sound will be about the same before and after reworking.

 

[mpinphilly]

Thanks everyone for responding here.  I was assuming the closest thing to these pres would the CL Labs stuff, which I've never heard but heard great things about...

For me the eye-opening point is the resale of these things.  Given my set up and how rarely I track more than a couple of tracks at a time, it may be a good idea to not chase more cash after these.  I have utter faith in Steve Hogan and it's pretty amazing someone with his credentials is offering these mods...  So that's one of the things that actually makes it harder to let these go--someone like Steve Hogan might be modding them! 

I also appreciate the suggestion to rotate the transformers.  Also, it's good to hear that these may not sound any different post-mod, save for being less noisy, but mainly will be more reliable... 

Part of the problem is that my ethos here is so utterly non-DIY or Lab-esque.  I really want to understand electronics, but I find it to be rather abstract...  I just need to get one of those electronic kits that kids get and start there.  So, for example, I know what a transformer even is, which right now I only know is something that gear has inside it (or doesn't).

thanks again!

 

[rattleyour]

I facilitated the purchase of six 73s, but as I'm living abroad right now I haven't heard them yet. Only two actually belong to me in the long run.

Aside from one DOA unit, my studio partner, who is himself a professional tech and accomplished AE, has been using three of the 73s quite happily alongside a host of vintage and boutique pres. I believe those three 73s (maybe four if he fixed the DOA) are actually in service in one of San Francisco's nicer studios.

The other two went to another AE friend of mine, and he's been using and loving them as well.

So ultimately I'll have to decide for myself, but I find it encouraging that two of my AE friends, whose ears I trust, are happy with their stock 73s.

 

[crazydoc]

Any news out there?

For anyone considering doing their own shielding, this looks like a suitable material, if used in layers, as it is thin.

http://www.goldmine-elec-products.com/prodinfo.asp?number=G16600A

 

[chris carter]

I e-mailed Steve last week, but no response.  I hope he's still working on these because right now he's my only hope of turning these 81s into more than doorstops 

 

[Paul G]

I expected a post from him last weekend, (see my post above).  I hope all is well.

;D

 

[Jeff Chitouras]

I received an email response from Steve this past weekend. He has been up to his eyeballs in finishing other work and expects to now be able to focus on the ACMP pre-amps he has received for upgrading.

 

[Paul G]

I was under the impression that Steve still has to check out the inductors.  This sounds like he's going to start modding units.

;D

 

[electric]

Where are people in the USA sourcing their BC441 and BC461's?

Also - do they need to be BC441-6/BC461-6?

I See Newark.com has BC441/BC461 (without the "-6"). Are these sufficient?

http://www.newark.com/magnatec/bc461/_/dp/25M7693
http://www.newark.com/magnatec/bc441/_/dp/96K6848?_requestid=62823

Regards,
Electric

 

[redddog]

Anyone heard from Mr. Hogan in a while?

 

[kazper]

Where are people in the USA sourcing their BC441 and BC461's?

Also - do they need to be BC441-6/BC461-6?

I See Newark.com has BC441/BC461 (without the "-6"). Are these sufficient?

http://www.newark.com/magnatec/bc461/_/dp/25M7693
http://www.newark.com/magnatec/bc441/_/dp/96K6848?_requestid=62823

Regards,
Electric

They will work, just lower gain rating for the lot.

 

[crazydoc]

Anyone heard from Mr. Hogan in a while?

Nope. I hope the bad karma that seems to surround these pres hasn't got him.

 

[Paul G]

Anyone heard from Mr. Hogan in a while?

Not in several weeks.

;D

 

[okgb]

Cinemag inductors are now on their way to me
will let you know

 

[wscrane]

I just got some 81s in my shop which have a severe hum problem with the EQ in, and was referred here.

As far as I can tell its the same as alexc described, and apparently fixed.  Alexc maybe you could confirm this ---  With the EQ out, the largest component is 180Hz  at -89dBV.  With the EQ in (and set to flat) the largest component is 120 Hz at -47dBV.  In both cases the output gain was 100%. 

Also, could you name your supplier for the Carnhills and mumetal cans?  Thanks.

Here's something I tried yesterday that didn't work, but its a data point...  One thing that bothered me about the circuit is that the output of each filter board has two grounds. Take for example the low presence board. One is 5R32 to the 24V ground.  The other is 5R36 to the 12V ground.  A difference in potential between these grounds will appear as signal. And I believe they arrive from separate long wires that are at opposite ends of a ribbon cable which forms a large loop right by the power transformer.  So... I lifted all the clip lite stuff off the signal path.  This includes 3R36, 4R32, 5R35, 6R35, and the two wire cable from board 6 to 7.  Funny thing is -- the hum got worse.  The 120Hz went to -39.5dBV.  So I think it is correct that the clip lite circuit is adding hum, but it turned out to be mostly antiphase and cancelling the main hum.   


   

 

[crazydoc]

I just got some 81s in my shop which have a severe hum problem with the EQ in, and was referred here.

As far as I can tell its the same as alexc described, and apparently fixed.  Alexc maybe you could confirm this ---  With the EQ out, the largest component is 180Hz  at -89dBV.  With the EQ in (and set to flat) the largest component is 120 Hz at -47dBV.  In both cases the output gain was 100%. 

Also, could you name your supplier for the Carnhills and mumetal cans?  Thanks.

Here's something I tried yesterday that didn't work, but its a data point...   One thing that bothered me about the circuit is that the output of each filter board has two grounds. Take for example the low presence board. One is 5R32 to the 24V ground.  The other is 5R36 to the 12V ground.  A difference in potential between these grounds will appear as signal. And I believe they arrive from separate long wires that are at opposite ends of a ribbon cable which forms a large loop right by the power transformer.  So... I lifted all the clip lite stuff off the signal path.  This includes 3R36, 4R32, 5R35, 6R35, and the two wire cable from board 6 to 7.  Funny thing is -- the hum got worse.  The 120Hz went to -39.5dBV.  So I think it is correct that the clip lite circuit is adding hum, but it turned out to be mostly antiphase and cancelling the main hum.     
 

Hi WS - welcome to the club (of which membership is unsought.  ;D  )

The absolute first thing to do is replace the Q4 and Q5 transistors on boards 3 thru 6. These oscillate and cause the major EQ noise component. Various substitutes have worked - I used 2N2222 and 2N2907 because I had some on hand - read thru the thread to see alternatives.

After that you can fiddle with the inductor noise. Also, rotating the toroid has help some folks, to a minor degree.

Alternative grounding schemes have been tried with little to no success.

There's a wealth if info here, especially from Steve Hogan, who seems to have dropped out of sight.

Schematics are at http://recordinghacks.com/tnc-audio-preamps/



Any suggestions/fixes from your end would be welcome.

 

[alexc]

wscrane,

As crazydoc says.

First replace the transistors - that will improve most of the problems. Maybe you can
live with it at this point. Remaining noise is in the low band, low frequencies, typically
at mains frequency and several harmonics.

If you can - replace the inductors with shielded ones. I used Carnhills from Audio Maintenance
and OEP shields from RS.

There's a fair bit of figuring the 'pin out' of the replacement inductor to the footprint
of the old one and mucking about with insulating and wiring and canning the Carnhills too.
I just wired and hot glued into place. This is probably not a very pro way to do it, so
maybe not suitable for someone else's unit. I put pics up earlier.

Cinemags are hopefully the best bet for rreplacements.

For sure, these 2 upgrades, almost exclusively, got rid of the excessive mains related noise, bringing it
to the level you would expect for a unit like this - which is pretty good!

Then, there is the loud pop on the gain switch. To workaround that, you can re-wire a resistor
and lose a switch position or some other more elaborate way to avoid it. I tend to remember to turn the gain all the way down if I'm changing the switch position and live with it.

---

My modded pair have been seeing use most nights as my lounge room guitar pre's. No probs to date
and they sound great. Teamed up with a dual g1176, and that's as good as I'm ever likely to hear!
I then send that combo to a Finalizer for recording and a tube hifi amp and speakers for monitoring.
The immediacy and depth of the sound is quite amazing.

 

[wscrane]

First replace the transistors - that will improve most of the problems. Maybe you can
live with it at this point. Remaining noise is in the low band, low frequencies, typically
at mains frequency and several harmonics.

Alexc, Thanks for the info.  I'm dubious about the transistor swap.  Could you be more specific about your particular experience?  Namely in what freq band is are the old transistors causing trouble and what kind of dB reduction did the swap accomplish?

 

[okgb]

the transistors cause an oscilation which
shows up as a broadband buzz / hum
definitely helps  , then you can see that the
hum left is more mains centered and you can use the eq if you
avoided those frequencies